Electric drive axle system and operating method

ABSTRACT

Methods and systems for an electric drive assembly are provided herein. In one example, a method for operation of an electric drive system is provided that includes coordinating operation of a first electric drive unit and a second electric drive unit based on a first power request. In the system, each of the first and second electric drive units include a planetary gear reduction that delivers power from a pair of motors to a set of axle shafts, and the planetary gear reductions have asymmetric gear ratios.

TECHNICAL FIELD

The present disclosure relates to a system with multiple electric driveaxles and a coordinated electric axle control method.

BACKGROUND AND SUMMARY

Electric drive systems, such as electric drive axles, have been providedin vehicles owing in part to their modularity. However, certain electricdrive axles have made performance tradeoffs between launch torque andcruising speed efficiency. Further, other drive axles have posedpackaging constraints on surrounding components or vice versa. Packagingmulti-motor drive axles between frame rails can prove difficult in somevehicle platforms, for instance. These packaging constraints and theinability to reach performance targets with regard to off the lineacceleration, in certain scenarios, may hinder the drive axle'sapplicability and ultimately customer appeal.

WO 2019/195229 A1 to David et al. teaches a powertrain where in oneembodiment multiple motors are coupled to a continuously variabletraction drive transmission. In this powertrain embodiment, two motorsare attached to a planetary gear reduction with a high ratio. Within theplanetary assembly, the sun and ring gears function as thetransmission's inputs and the carrier functions as the transmission'soutput.

The inventors have recognized several drawbacks with the powertrainstaught by David. For instance, the above described powertrain embodimentmay demand tradeoffs to be made with regard to vehicle launch andcruising performance, due to system architecture. As an example, therange of transmission ratios in David's powertrain may be selected tofavor off the line acceleration at the expense of high speed cruisingefficiency or vice versa. Further, David is silent with regard to thepowertrain's specific end-use component layout. In practice, David'ssystem and other prior powertrains may therefore impose spaceconstraints on surrounding vehicle components (e.g., the frame, batterysystem, passenger cabin, auxiliary systems, etc.) in certain vehicleplatforms. These space constraints may precipitate modification orredesign of certain vehicle systems to accommodate for the spaceinefficient transmission. Vehicle production times and costs may beresultantly driven up.

To address at least a portion of the abovementioned issues, a method foroperation of an electric drive system is provided. The method comprises,while the system is operated under a first condition, coordinatingoperation of a first electric drive unit and a second electric driveunit based on a first power request. In the system, the first electricdrive unit includes a first planetary gear reduction that delivers powerfrom a first pair of motors to a first set of axle shafts. Further inthe system, the first electric drive unit includes a second planetarygear reduction that delivers power from a second pair of motors to asecond set of axle shafts. Still further in the system, the first andsecond electric drive units have asymmetric gear ratios. For instance,the drive units may be controlled in tandem to achieve high torquelaunch. In this way, the drive system's performance may be increased.

The system's control method may further comprise disconnecting the firstelectric drive unit through operation of a disconnect clutch. In thisexample, the first electric drive unit has a higher gear ratio than thesecond electric drive unit. Operating the disconnect clutch in thismanner enables the second electric drive unit to be efficiently operatedduring cruise operation without the first electric drive unit creatingpowertrain drag. Consequently, the system's efficiency during cruiseoperation may be increased.

In another example, the degree of disengagement of disconnect clutch maybe adjusted according to efficiency set-points of the motors in each ofthe first and second drive units or according to a desired load sharingfraction corresponding to the drive units. In this way, the efficiencyof the system may be further increased to enable the vehicle's range tobe extended, for instance.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration of a vehicle with an electric drive system.

FIG. 2 is a first example of a multi-motor drive unit.

FIG. 3 is a second example of a multi-motor drive unit.

FIG. 4 is a control method for an electric drive system.

FIGS. 5-6 are graphical representations of different use-case electricdrive system control techniques.

FIG. 7 is a timing diagram of a use-case electric drive system controlstrategy.

FIGS. 2-3 are drawn approximately to scale. However, alternate relativedimensions may be used, in other embodiments.

DETAILED DESCRIPTION

An electric drive system that achieves a space efficient arrangement andenhances vehicle launch and cruising performance is described herein.The drive system includes a tandem axle, with each axle having amulti-motor drive unit with asymmetric gear ratios. Within theasymmetric gear ratios, the drive units may have continuous variability,in some cases. Because of this gear ratio variability, the drive unitscan avoid performance tradeoffs and efficiently realize acceleration andother performance targets. For instance, the drive unit with the highergear ratio allows for greater launch torque. Conversely, the lower gearratio drive unit allows the powertrain to attain greater efficiency atcruising speeds and lighter loads. Further, in one example, the highergear ratio drive unit may have a disconnect clutch. In such an example,the disconnect clutch may decouple the higher ratio drive unit to permitthe other drive unit to be efficiently operated at a higher speed duringvehicle cruise operation, for instance. Other coordinated controlstrategies may be implemented using disconnect clutch operation, such asclutch operation for load sharing to increase system efficiency.

Each of the drive units in the electric drive system may have a side byside motor layout, in one example, or a longitudinal motor layout, inanother example. In this side by side motor layout, the motors areadjacent to one another with parallel rotational axes, and in thelongitudinal layout, the motors are coaxial. The side by side layoutallow for tight package space installations between tandem axles. On theother hand, the longitudinal motor layout allows for tight package spaceinstallations between the frame rails. Packaging the drive unit betweenthe frame rails may decrease the likelihood of drive unit degradationfrom road debris and/or other objects in the vehicle's operatingenvironment. In this way, the system can be adapted for a variety ofvehicle platforms. The enhanced adaptability may increase manufacturingand servicing efficiency as well as customer appeal.

FIG. 1 illustrates a vehicle 100 with a powertrain 102. The vehicle 100may be a light, medium, or heavy duty vehicle designed for on and/oroff-road travel. The vehicle 100 includes powertrain 102 with anelectric drive system 104. The vehicle 100 is further illustrated withan unpowered steerable axle 106 (e.g., steerable front axle). In otherexamples, the steerable axle 106 may be powered via an internalcombustion engine. As such, the vehicle may be a battery electricvehicle (BEV), in one example, or a hybrid electric vehicle, in otherexamples.

The electric drive system 104 comprises a first multi-motor drive unit108 and a second multi-motor drive unit 110 in a tandem axle assembly112 (e.g., non-steerable tandem axle assembly). However, in alternatearrangements, the drive units may be spaced away from one another. Themulti-motor drive units may be more generally referred to as electricdrive units.

A dependent suspension system 114 may further be provided in the vehicle100, in some instances. Using dependent suspension allows the vehicle'sdurability and load carrying capacity to be increased when compared toindependent suspension systems. Springs 116 coupled to axles 118, 120may therefore be provided in the suspension system. However, other typesof suspension systems may be used, in other examples, which may howeverdecrease vehicle durability and load carrying capacity. Further, theaxles 118, 120 may be beam axles in which the drive wheels are connectedvia a continuous beam or shaft.

The first multi-motor drive unit 108 includes a first motor 122 and asecond motor 124 attached to a planetary gearset 126 (e.g., planetarygear reduction). As such, the first multi-motor drive unit may bereferred to as a dual-motor drive unit, in one example. However, thefirst multi-motor drive unit may include three or more motors, in otherexamples. The motors 122, 124 as well as the other motors describedherein may include rotors that electromagnetically interact with statorsto generate mechanical and electric power, in some cases. As such, themotors may be motor-generators. The first and second motors 122, 124 mayhave different characteristics (e.g., size, peak power, maximum speeds,efficiency curves, etc.). In this way, the drive unit may blend motorperformance characteristics to, for example, realize performance andefficiency goals. Alternatively, the motors in the first multi-motordrive unit 108 may be similar in size, which may however decrease theunit's performance and/or efficiency. Further, as described herein, amotor is a device with components (e.g., rotor, stator, housing, and thelike) for generating rotational energy or, in the case of amotor-generator, receiving rotational energy and transforming it intoelectrical energy. The motors described herein may be alternatingcurrent (AC) type motors such as multi-phase motors. However, directcurrent (DC) type motors may be deployed in certain scenarios (e.g., lowspeed and low power vehicle platforms).

The planetary gearset 126 may be a simple planetary gearset with a ringgear 128, planet gears 130 that rotate on a carrier 132, and a sun gear134. Thus, the planetary gearset 126 and the other planetary gearsetsdescribed herein may not have clutches or brakes to simplifyconstruction and increase drive unit compactness. However, more complexplanetary assemblies may be used, in other embodiments, which may maketradeoffs with regard to space efficiency. For instance, the planetarygearset 126 may include additional gears such as multiple sets of planetgears and/or ring gears, in alternate embodiments. In one particularexample, the first drive unit may include two planetary gearsets whichhave ring gears and carriers that mesh with one another or a multi-stageplanetary gearset. These planetary variants are also applicable to aplanetary gearset 158 included in the second multi-motor drive unit 110.The planetary gearsets 126, 158 may include toothed gears which meshwith one another, in one example. Alternatively, the planetary gearsets126, 158 and/or the other planetary assemblies described herein may betraction drives. In this traction drive example, the planetaryassemblies may include sun, ring, and planet components whose mode ofpower transfer is through shear forces in a relatively thin layer offluid trapped between the components. Further, as described herein, thetraction drives may additionally use friction as another mode of powertransfer, in some cases.

In the illustrated example, the first and second motors 122, 124 and theplanetary gearset 126 are coaxial. Rotational axes 136 of the motors122, 124 that are coaxial with the planetary gearset 126 are providedfor reference. However, alternate motor and planetary gearsetarrangements have been envisioned. The motor and planetary layout may bechosen based on packaging constraints imposed by other vehicle systems(e.g., the suspension system, frame, battery system, etc.), forinstance. As such, when longitudinal compactness is favored, the motorsmay have a coaxial layout. Conversely, when used in a tandem axle andlongitudinal compactness is favored, the motors may have a side by sidelayout, described in greater detail herein with regard to FIG. 3.

Further, in the illustrated example, the rotational axes of the motorsand the planetary gearset are perpendicular to the rotational axis 177of the first axle 118. In an alternate example, the motors and theplanetary gearset may be arranged coaxial to one another and parallel tothe first axle.

In the planetary gearset 126, the ring gear 128 is coupled to an outputshaft 138 of the first motor 122. Further, the sun gear 134 is coupledto an output shaft 140 of the second motor 124. Therefore, the ring andsun gears may serve as the inputs for the planetary assembly. On theother hand, the carrier 132 may serve as the planetary assembly'soutput. To elaborate, a gear 142 may be coupled to the carrier 132 whichfunctions as a power interface for downstream components.

An output shaft 144 with a first gear 146 that meshes with the gear 142and a second gear 148 that meshes with an output gear 150 are furthershown in FIG. 1. Arrows 149 denote the mechanical connection (e.g.,mesh) formed between the output gear 150 and the second gear 148. In oneexample, the mechanical connection between the gears 148, 150 may takethe form of a hypoid gear reduction with gears that are perpendicular toone another. Alternatively, the mechanical connection formed between thegears 148, 150 may take the form of a helical gear reduction. In eithercase, the gears 148, 150 may function as a final drive arrangement. Afinal drive refers to the last gear reduction in the drivetrain upstreamof a corresponding differential. Although, the first and secondmulti-motor drive units 108, 110 are illustrated longitudinally forwardof the drive axles 118, 120, respectively, at least a portion of eachdrive unit may be arranged longitudinally rearward of the drive axles,in other embodiments. For instance, the motors 122, 156 may be arrangedrearward of the axles 118, 120, respectively.

Further, the output gear 150 may be coupled to a first differential 152.Arrows 151 indicate the mechanical connection between the firstdifferential 152 and the output gear 150. Open differentials, lockingdifferential, and limited slip differentials have been contemplated foruse in the powertrain. The type of differential used in the system maybe selected based on end-use traction performance objectives. Furtherthe first differential 152 is included in the first axle 118 thatadditionally includes axle shafts 153. In turn, the axle shafts 153 arerotationally coupled to drive wheels 155.

The second multi-motor drive unit 110 may have certain similarities withthe first multi-motor drive unit 108. For instance, the secondmulti-motor drive unit 110 again includes a first motor 154, a secondmotor 156, a planetary gearset 158 (e.g., planetary gear reduction), ashaft 159, and an output gear 162. Again, the first and second motorsmay be asymmetrically sized to increase drive unit performance andefficiency, although drive unit configurations employing similarly sizedmotors have been envisioned. Further, when the second multi-motor driveunit 110 includes two motors it may be referred to as a dual-motor driveunit, in one example. Alternatively, in other examples, the secondmulti-motor drive unit may include three or more motors.

Although the multi-motor drive units may have a common componentarchitecture, the ratios of the drive units are asymmetric. Toelaborate, the second multi-motor drive unit 110 has a smaller gearratio than the first multi-motor drive unit 108. In one specificexample, the second multi-motor drive unit may have a ratio that is lessthan 10:1 (e.g., 7:1) and the first multi-motor drive unit may have aratio greater than 10:1 (e.g., 12:1). This asymmetric gear ratio betweenthe drive units allows each unit to have separate performance andefficiency characteristics that can be blended to achieve a moreexpansive set of performance and efficiency targets in the system. Forexample, the first drive unit may generate higher torque at low speed,such as during vehicle launch, and the second drive unit may operatewith greater efficiency at higher cruising speeds. In this way, thevehicle can not only accelerate more quickly off the line but alsoachieve greater efficiency while cruising. Consequently, both vehicleperformance and efficiency can be enhanced across a wider range ofoperating conditions. The control strategies of the drive units arediscussed in greater detail herein with regard to FIGS. 4-7.

The planetary gearset 158 may include a ring gear 160, planet gears 161rotating on a carrier 164, and a sun gear 166. Thus, the planetarygearset 158 may be a simple planetary gearset. Although, as noted above,the planetary gearset may have a more complex layout. Again, the ringgear 160 and the sun gear 166 may function as the planetary assembly'sinputs and the carrier 164 may function as the planetary output with agear 167 coupled thereto. Further, gears 170, 172 reside on shaft 159which mesh with gear 167 and the output gear 162, respectively. Arrows171 denote the mechanical connection formed between the output gear 162and the gear 170. Again, the mechanical connection may be a hypoid orhelical gear mesh.

The output gear 162 is rotationally coupled to a second differential174, as indicated via arrows 176. The differential 174 may be an opendifferential, locking differential, limited slip differential, and thelike. Further, the differential 174 is included in the axle 120, whichincludes axle shafts 178. In turn, the axle shafts are rotationallycoupled to drive wheels 180.

The first and second multi-motor drive units 108, 110 each have similarcoaxial layout of the motors and planetary gearsets. However, otherdrive unit arrangements, discussed herein with regard to FIGS. 2-3, havebeen envisioned.

Each of the first and second multi-motor drive units 108, 110 may becontinuously variable (e.g., infinitely variable). As such, each driveunit may have a continuous range (e.g., infinite range) of gear ratioswithin the drive unit's overall gear ratio range. To achieve thecontinuous variable ratio adjustability, continuous speed and/or torquecontrol of the motors 122, 124 and/or motors 154, 156 may be coordinatedto reach a ratio set-point. Gains in powertrain efficiency and vehicleperformance are achieved when the drive units are designed forcontinuous variability. However, other types of drive units may be used,in other embodiments, which may however demand trade-offs with regard toefficiency and performance, in some cases.

The motors in each multi-motor drive unit may receive electrical energyfrom an energy storage system 182 (e.g., a battery system). The energystorage system may include one or more energy storage devices 184 (e.g.,batteries, capacitors, flywheels, and the like). The vehicle may furtherinclude an inverter between the batteries and the drive unit motors,when the motors 122, 124, 154, 156 are multi-phase motors or othersuitable type of AC motors.

Further in one example, a disconnect clutch 186 may be arrangeddownstream of the planetary gearset 158. For instance, the disconnectclutch 186 may be coupled to the output gear 162 as illustrated.Arranging the disconnect clutch adjacent to the output gear permits asmaller number of components to be rotating with the output when thedisconnect is active in comparison to systems with the disconnect clutcharranged further upstream in the multi-motor drive unit. However, inother examples, the disconnect clutch may be arranged at a differentlocation, such as further upstream in the multi-motor drive unit. Setsof plates 188 may be included in the disconnect clutch 186 to accomplishthe power transfer between the second multi-motor drive unit 110 and theaxle 120. In this way, the disconnect clutch may be engaged anddisengaged to permit and inhibit power transfer between the componentsit is rotationally coupled to.

A control system 190 with a controller 192 may further be incorporatedin the powertrain 102. The controller 192 includes a processor 194 andmemory 196. The memory 196 may hold instructions stored therein thatwhen executed by the processor cause the controller 192 to perform thevarious methods, control strategies, etc., described herein. Theprocessor 194 may include a microprocessor unit and/or other types ofcircuits. The memory 196 may include known data storage mediums such asrandom access memory, read only memory, keep alive memory, dual-memory,combinations thereof, etc.

The controller 192 may receive vehicle data and various signals fromsensors positioned in different locations in the electric drive system104 and/or the vehicle 100. The sensors may include motor speed sensors181, motor temperature sensors 183, disconnect clutch sensor 185, wheelspeed sensors 187, and the like. In this way, gear speed at the inputand the output of the system may be detected along with the gear speedat the output of the first planetary gearset 148. However, in otherexamples, the speeds of at least a portion of the gears may be modeledby the controller.

The controller 192 may send control signals to actuators in the motors122, 124, 154, 156. Responsive to receiving the control signals, motorspeed may be adjusted, for instance. Further, control commands may besent to the disconnect clutch 186. The other controllable components inthe electric drive system may function similarly with regard to controlcommands and actuator adjustment. The electric drive system 104 mayinclude an input device 198 (e.g., an accelerator pedal, acontrol-stick, levers, buttons, combinations thereof, and the like). Theinput device 198, responsive to operator input, may generate a vehiclepower request.

An axis system is provided in FIG. 1 as well as FIGS. 2-3 for reference.An x-axis, y-axis, and z-axis are specifically depicted. The x-axis maybe a lateral axis, the y-axis may be a longitudinal axis, and the z-axismay be a vertical axis (e.g., an axis parallel to the gravitationalaxis). However, different orientations of the axes may be used, in otherexamples. Rotational axes 177, 179 of the axles are further provided forreference.

FIG. 2 illustrates a first example of a multi-motor drive unit 200. Themulti-motor drive unit 200 may specifically serve as an example of oneof the drive units 108, 110 depicted in FIG. 1. As such, the drive units108, 110, shown in FIG. 1, and the drive unit 200 shown in FIG. 2 mayshare common structural and/or functional features. In certainconfigurations, the drive units 108, 110, shown in FIG. 1, may each havean architecture similar to the multi-motor drive unit 200 shown in FIG.2. Redundant description is omitted for brevity.

The multi-motor drive unit 200 includes a first rotor shaft 202 that isincluded in a first motor 203 and a second rotor shaft 204 that isincluded in a second motor 205. It will be appreciated that the motorsinclude rotors electromagnetically interacting with stators. As shown inFIG. 2, both rotor shafts may be designed to rotate in opposingdirections, indicated via arrows 207, 209. Specifically, in one example,during drive operation, the motors may be rotated in oppositedirections. In this way, the drive generated by the units issymmetrically reversible, and can seamlessly change directions whendesired.

Bearings 206, 208 may be coupled to the rotor shafts 202, 204. Thebearings described herein may include races and rotational elements(e.g., balls, cylinders, tapered cylinders, etc.) that support andconstrain rotation of the component(s) to which they are attached. Afirst planetary gearset 210 may be coupled to the first rotor shaft 202.The planetary gearset 210 may include ring gear 212, planet gears on acarrier, and a sun gear. Bearings 214 may be positioned on opposingsides of the planetary gearset 210.

The first rotor shaft 202 is coupled to the sun gear, and the ring gear212 is coupled to a connection shaft 216. The connection shaft 216extends between the first planetary gearset 210 and a second planetarygearset 218. The second planetary gearset 218 includes a ring gear 220,planet gears 222 on a carrier 224, and sun gear 226.

Two bearings 228 are coupled to the second planetary gearset 218.Further, a gear 230 is coupled to the carrier 224. The second rotorshaft 204 is coupled to the sun gear 226. In this way, the sun and ringgears function as the planetary gearset's inputs and the carrierfunctions as the output.

A shaft 232 with a pinion gear 234 and an output gear 236 (e.g., hypoidgear) may further be included in the multi-motor drive unit 200.However, other gear arrangements may be used, in other examples. Thepinion gear 234 meshes with the gear 230 and the output gear 236 may becoupled to downstream components such as the differential or a shaftfunctioning as the differential's input. The differential's rotationalaxis may be arranged perpendicular to the shaft 232. Further, bearings238, 240 may be coupled to the shaft 232. The output gear 236 may beradially offset from the motors 203, 205 and the planetary gearsets 210,218. In this way, the output gear may be positioned to efficientlyattach to the axle differential.

The rotor shafts 202, 204 are arranged coaxial to one another, in theillustrated example. In this way, the motors are longitudinallyarranged. Because of the longitudinal arrangement, the drive system maybe more efficiently packaged in the vehicle. For instance, thelongitudinal arrangement may be used to fit the drive unit between twoopposing frame rails 242, 244. Consequently, in one use-case example,the drive unit may be placed higher in the vehicle when compared tovehicles the electric drive systems that are wider than the frame, whichmay demand unit placement below the frame rails. The higher placement ofthe drive unit makes it less susceptible to degradation from interactionwith objects such as road debris and may be more space efficientlyincorporated into a wide variety of vehicle platforms.

Further, as illustrated, the output gear 236 is offset from therotational axes 246, 248 of the rotor shafts 202, 204. Further, therotational axes 246, 248 are coaxial and coaxial with the centralrotational axes of the planetary gearsets 210, 218. The output gear 236may further be interposed by the motors 203, 205 (e.g., housings of themotors). In other words, the motors may be positioned on axial opposingsides of the output gear 236. In this way, downstream components can beeasily connected to the multi-motor drive unit while retaining a spaceefficient layout. The layout depicted in FIG. 2 allows the multi-motordrive unit 200 to compactly be arranged between frame rails 242, 244 andtherefore positioned higher in the vehicle to guard the unit againstdegradation from road debris and other environmental factors.

A rotational axis 250 of the shaft 232 is further provided forreference. As shown, the shaft may be rotated in opposing rotationaldirections, as indicated via arrows 252. The rotational axis 250 may beparallel to the rotational axes 246, 248. Further, as shown in FIG. 2,the output gear 236 is axially offset from the planetary gear set 218.

FIG. 3 shows a second example of a multi-motor drive unit 300. Themulti-motor drive unit 300 may specifically serve as an example of oneof the multi-motor drive units 108, 110 depicted in FIG. 1. In certainconfigurations, the multi-motor drive units 108, 110, shown in FIG. 1,may each have a component architecture similar to the multi-motor driveunit 300, shown in FIG. 3. However, in other examples, one of themulti-motor drive units 108, 110 shown in FIG. 1 may have anarchitecture similar to the multi-motor drive unit 300 depicted in FIG.3 and the other unit may have a similar architecture to the multi-motordrive unit 200, shown in FIG. 2. Alternatively, to simplifymanufacturing and allow for commonality of components, and installationinterfaces in the vehicle, the vehicle may include two of themulti-motor drive units 200 or 300, shown in FIGS. 2 and 3,respectively.

The multi-motor drive unit 300 again includes a first rotor shaft 302 ina first motor 304 and a second rotor shaft 306 in a second motor 308.However, the rotational axis 310, 312 of the rotor shafts 302, 306 areparallel to one another. Thus, the motors are arranged adjacent andoffset from one another, referred to herein as a side by side layout.Because of the side by side layout the drive unit, the unit may be usedwhere tight package space installation longitudinally between the tandemaxles is desired. The motors 304, 306 may be positioned laterallybetween two frame rails to achieve space efficient installation.

The rotor shafts 302, 306 are each coupled to sun gears 314, 316 inplanetary gearsets 318, 320, respectively. The planetary gearsets 318,320 may be simple planetary gearsets with the ring gears 322, 324 thatinclude teeth 326, 328 on outer surfaces that mesh with one another.Alternatively, in the traction drive embodiment, the outer surfaces ofthe ring gears may engage with one another via shear forces and/orfriction. However, alternate gear arrangements have been contemplated.For instance, a gear coupled to a motor output shaft 330 may directlymesh with the outer teeth 328 of the ring gear 322. It will beappreciated that the planetary gearsets 318, 320 further include planetgears 332, 334 and carriers 336, 338. Power from the carriers 336, 338is combined via gears 340, 342 coupled to the carriers and mesh with oneanother via outer teeth.

Bearings 344 (e.g., ball bearings) are coupled to the shafts on whichthe gears 340, 342 rotate. In this way, the planetary assemblies areefficiently supported. The shaft 346 on which the gear 340 rotatesextends between the planetary gearset 318 and a gear 348. Again,bearings 350 may be provided on either side of the gear 348 to furthersupport the shaft 346.

An output gear 352 resides on a shaft 354. During drive operate, theshaft 354 receives power from a mesh between the gear 348 and a gear356. Bearings 358 may be coupled to the shaft 354. A mesh between theoutput gear 352 (e.g., hypoid gear) and a downstream gear coupled to adifferential may form a final drive of the multi-motor drive unit.Although, other powertrain layouts may be used. Further, the motors 304,306 may be positioned on an inboard axial side 360 of the output gear352, to increase the unit's packaging efficiency while still permittingthe unit to be quickly attached to a differential.

The layouts depicted in FIGS. 2-3 strike a desired balance between motorpackaging, gearing compactness, gear ratio range, and drive unitperformance. For instance, the drive unit embodied in FIGS. 2 and 3 haveprofiles that may efficiently fit between frame rails. Further, thedrive unit embodied in FIG. 3 has a profile that enables the drive unitsto be spaced relatively close together along a longitudinal axis. Inthis way, these drive units can meet differing packaging goals in awider variety of vehicle platforms. Consequently, the applicability ofthe drive units may be expanded to meet design constraints in amultitude of vehicle types, thereby increasing customer appeal.

FIG. 4 shows a method 400 for operation of an electric drive system. Themethod 400 may be carried out by the electric drive systems andcomponents described above with regard to FIGS. 1-3, in one example.However, in alternate examples, the method 400 may be implemented usingother suitable electric drive systems and corresponding components.Further, the method 400 may be carried out as instructions stored inmemory executed by a processor in a controller. As such, performing themethod steps may include receiving signal input as well as sendingand/or receiving commands which trigger adjustment of associatedcomponents, as previously indicated.

At 402, the method includes determining operating conditions such asvehicle speed, an operator power request, motor speed, motor torque,motor temperature, etc. These operating conditions may be ascertainedusing sensor inputs and/or modeling.

Next at 404, the method judges whether a launch condition is present.Such a judgement may be carried out using vehicle speed and/or theoperator power request. As such, if the vehicle speed is at zero or anear zero value and a power request has been received, it may beascertained that a launch condition is present. Conversely, if thevehicle speed is a non-zero value, a power request has not beenreceived, and/or vehicle brakes are engaged, a launch condition is notoccurring. Different launch conditions may be delineated via a non-zeropower request threshold. For instance, if the operator power request isgreater than the threshold, the launch condition may be characterized asa first launch condition in which dual drive unit operation may betriggered. On the other hand, if the operator power request is less thanthe threshold value, the launch condition may be characterized as asecond launch condition in which single drive unit operation may betriggered.

If it is judged that a launch condition is present (YES at 404), themethod moves to 406. At 406 the method determines if the power requestis greater than a threshold value. This threshold value may separatesingle and dual drive unit operation during launch. The threshold valuesmay be a predetermined or dynamic non-zero value and may be calculatedbased on motor size, motor efficiency objectives, vehicle weight,battery state of charge, and the like.

When the power request is below the threshold (YES at 406) the methodadvances to 408, where the method includes operating the second electricdrive unit while the first electric drive unit remains deactivated. Step408 may therefore include operating the first drive unit based on atorque set-point at 409 and inhibiting operation of the second driveunit at 410. Deactivation of the first drive unit may involve operationof the unit in a coast mode where the motors are not generatingmechanical or electrical power. As such, during deactivation of thefirst drive unit, electrical energy transfer to the drive unit maycease. In yet another example, a disconnected clutch in the first driveunit may be disengaged during deactivation. When controlling the seconddrive unit, one or both of the motors in said unit may be operated in atorque or speed control mode to meet the operator's power request. Forinstance, the first motor may be operated at a torque set-point whilethe second motor is operated at a speed set-point, which maintains thefirst motor at a target speed or within a speed range. Further, in othercases, both the first and second motors in the first drive unit may beoperated at different speed set-points.

However, if the power request is greater than the threshold value (NO at406) the method moves to 411. At 411, the method includes cooperativelyoperating the first and second drive units. Step 411 may thereforeinclude operating the first and second drive units based on a torqueset-point, at 412. For instance, both the first and second units may beoperated to generate additive forward drive. As such, each drive unitmay fractionally contribute to drive wheel power and said fraction maybe altered in response to changes in vehicle load, speed, motorefficiency, and battery state of charge, for example. To elaborate, thecontribution of each drive unit may be adjusted based on efficiencyset-points, the gear ratio of each unit, vehicle traction, and the like.For instance, both units may be operated to maintain each of the pairsof motors within a desired efficiency range to extend vehicle range.Again, both motors in each drive unit may be speed or torque controlledaccording to the power request. For example, one motor may be torquecontrolled while the other is speed controlled or vice versa.

Further, in examples, where the system includes the disconnect clutch inthe higher ratio electric drive unit, load sharing strategies may bedeployed that further enhance different aspects of drive system andvehicle performance such as efficiency, handling, responsiveness, etc.As such, the disconnect clutch may be partially disengaged for loadsharing efficiency. The clutch may therefore increase its degree ofengagement to increase load on the second drive unit and vice versa.Thus, the clutch may be operated to maintain a load splitting ratiobetween the electric drive units. In other cases, the disconnect clutchmay be disengaged to reduce drive system drag. In yet another example,the disconnect clutch may be selectively disengaged to reduce (e.g.,minimize) energy consumption of the drive units. The energy consumptiontargets may be modeled using battery state of charge, vehicle load,motor temperatures, motor speeds, and the like.

Conversely, if it is judged that a launch condition is not present (NOat 404) the method moves to 413. At 413, method judges if the vehicle isoperating under a cruise condition. For instance, it may be judged thata cruise condition is occurring when the vehicle speed is greater than athreshold value, the operator power request is less than a thresholdvalue, and/or the load is less than a threshold value. It will beunderstood that the aforementioned threshold values may be positivenon-zero values. On the other hand, when the vehicle speed is less thanthe threshold value, the operator power request is greater than thethreshold value, or the load is greater than the threshold value it maybe judged that the cruise condition is not occurring.

If it is judged that vehicle cruise is not occurring (NO at 413), themethod moves to 414. At 414, the method includes controlling theelectric drive units according to a current operating strategy. Thecurrent operating strategy may involve the adjustment of both electricdrive units in tandem based on operator power request and vehicle loador may involve sustaining the drive units in shut down when the vehicleis in park mode. To elaborate, operating both units in tandem mayinvolve operating the first and second electric drive units concurrentlyto achieve an operator requested power set-point.

Conversely, the method proceeds to 416 when it is judged that vehiclecruise is occurring (YES at 413). At 416, the method includes operatingthe first drive unit while the second drive unit is inoperative, whichmay include disconnecting the second drive unit via the disconnectclutch, at 418. Additionally, the electrical power transfer the seconddrive unit may be terminated at step 416. It will be appreciated thatboth steps 408, 410, and 416 may be generally characterized ascoordinating operation of the first and second electric drive units.After 408, 411, and 416, the method ends. Method 400 allows the system'soperating strategy to be tailored to achieve a high torque vehiclelaunch and an efficient cruise operation.

Referring to FIGS. 5 and 6, graphical relationships between an operatorpower request and vehicle speed that are used in different drive systemcontrol strategies, are shown. The relationships shown in FIGS. 5 and 6are for illustrative purposes only and are not meant to be limiting. Theordinates represent operator power request, and the power requestincreases in the direction of the ordinate arrows. The abscissasrepresent vehicle speeds, and vehicle speed increases in the directionof the abscissa arrows.

Turning specifically to FIG. 5, the drive system control strategycorresponds to an electric drive system that utilizes a disconnectclutch in the higher gear ratio drive unit (e.g., the second multi-motordrive unit 110 shown in FIG. 1). Graphical region 500 indicates theranges of vehicle speeds and power request values where the higher ratiodrive unit may be operated for efficiency. In this way, efficiency gainsmay be achieved during low speed operation while power demand isrelatively low.

Graphical region 502 indicates the ranges of vehicle speeds and powerrequest values where both drive units are operated when a higher powerrequest is present. As such, both drive unit are operated in tandem torapidly achieve a higher power request, thereby increasing drive systemresponsiveness.

Graphical region 504 indicates the ranges of vehicle speeds and powerrequest values where the higher ratio drive unit is disconnected and thelower ratio drive unit is operated. By disconnecting the higher ratiodrive unit, the system efficiently operates at a higher speed.

Plot 506 indicates the threshold power at which the system transitionsfrom single drive unit operation to multi drive unit operation or viceversa. For instance, when the operator power request surpasses the powerthreshold, the system switches to a dual drive unit control mode.Conversely, when the power request falls below the threshold, the systemtransitions to a single unit control mode where the higher ratio driveunit is solely operated.

Plot 508 indicates the threshold speed at which the system transitionsto a single unit control mode where the lower ratio drive unit is solelyoperated while the higher ratio drive unit is disconnected. As such,when the vehicle speed surpasses the threshold value 508, the system maytransition from the dual unit control mode to the single unit controlmode where the higher ratio drive unit is disconnected or vice versa. Inanother instance, the system may transition from the single unit controlmode where the higher ratio drive unit is solely operated to the othersingle unit control mode where the lower ratio drive unit is solelyoperated or vice versa.

Turning specifically to FIG. 6, the drive system control strategycorresponds to an electric drive system where a disconnect clutch isomitted. Graphical region 600 indicates the ranges of vehicle speeds andpower request values where the higher ratio drive unit may be operatedfor efficiency. Conversely, graphical region 602 indicates the ranges ofvehicle speeds and power request values where both drive units areoperated to meet a higher power demand. Plot 604 indicates the thresholdpower at which the system transitions from single drive unit operationto multi drive unit operation or vice versa.

The control strategy embodied in FIG. 6 may be deployed when systemsimplicity is favored while the control strategy embodied in FIG. 5 maybe deployed when system performance is desired. As such, the system maybe tailored to meet a variety of end-use design characteristics.

FIG. 7 shows a timing diagram 700 of a use-case control strategy for anelectric drive system, such as any of the electric drive systems shownin FIGS. 1-3. However, a similar control strategy may be used to operateother suitable electric drive systems. In each graph of the timingdiagram, time is indicated on the abscissa, increasing in the directionof the arrow. The ordinates for plots 702, 704, 706 indicate vehiclespeed, vehicle load, and operator power demand, respectively. Vehiclespeed, vehicle load, and operator power demand increases in thedirection of the ordinate arrow. The ordinates for plots 708, 710indicate the operating states (operational and non-operational) of thehigher ratio electric drive unit and the lower ratio electric driveunit, respectively.

At t1, operator power demand surpasses a threshold value 712. Thisthreshold value may be a non-zero value that is calculated based onmotor size, motor efficiency, drive unit gear ratios, final drive ratio,vehicle weight, combinations thereof, and the like. Further, thethreshold power demand may be dependent upon vehicle speed, in oneexample. Alternatively, the threshold power demand may be a constantvalue that is predetermined. Responsive to the power demand exceedingthe threshold value, the lower ratio drive unit transitions to anoperational state. Thus, from t1 to t2, both of the electric drive unitsare online and generating mechanical power.

At t2, vehicle speed surpasses a threshold value 714 indicative ofcruise operation. The threshold speed value may be a non-zero value thatis calculated based on motor size, motor efficiency, drive unit gearratios, final drive ratio, vehicle weight, combinations thereof, and thelike. In one example, the threshold speed may be a constant value. Thesystem disconnects the higher ratio electric drive unit in reaction tothe vehicle speed surpassing the threshold value. In this way, when acruising speed is attained, one of the drive units may be decoupled fromthe axle shafts to increase system efficiency. Further, the higher ratiodrive unit may be permitted to disconnect when the vehicle load is lessthan a threshold value 716, which again is a non-zero value.Alternatively, when the vehicle load is greater than the threshold 716,the system may inhibit disconnection of the higher ratio drive unit. Inthis way, both drive units may be operational while the vehicle loadremains relatively high.

The technical effect of the electric drive system control methodsdescribed herein is to increase system efficiency and performance withregard to responsiveness and off the line acceleration using coordinatedoperation of the drive units.

FIGS. 1-3 show example configurations with relative positioning of thevarious components. If shown directly contacting each other, or directlycoupled, then such elements may be referred to as directly contacting ordirectly coupled, respectively, at least in one example. Similarly,elements shown contiguous or adjacent to one another may be contiguousor adjacent to each other, respectively, at least in one example. As anexample, components laying in face-sharing contact with each other maybe referred to as in face-sharing contact. As another example, elementspositioned apart from each other with only a space there-between and noother components may be referred to as such, in at least one example. Asyet another example, elements shown above/below one another, at oppositesides to one another, or to the left/right of one another may bereferred to as such, relative to one another. Further, as shown in thefigures, a topmost element or point of element may be referred to as a“top” of the component and a bottommost element or point of the elementmay be referred to as a “bottom” of the component, in at least oneexample. As used herein, top/bottom, upper/lower, above/below, may berelative to a vertical axis of the figures and used to describepositioning of elements of the figures relative to one another. As such,elements shown above other elements are positioned vertically above theother elements, in one example. As yet another example, shapes of theelements depicted within the figures may be referred to as having thoseshapes (e.g., such as being circular, straight, planar, curved, rounded,chamfered, angled, or the like). Additionally, elements co-axial withone another may be referred to as such, in one example. Further,elements shown intersecting one another may be referred to asintersecting elements or intersecting one another, in at least oneexample. Further still, an element shown within another element or shownoutside of another element may be referred as such, in one example. Inother examples, elements offset from one another may be referred to assuch.

The invention will be further described in the following paragraphs. Inone aspect, an electric drive system is provided that comprises a firstmulti-motor drive unit including a first planetary gear reduction thattransfers power between a first pair of motors and a first set of axleshafts; and a second multi-motor drive unit including a second planetarygear reduction that transfers power between a second pair of motors anda second set of axle shafts; wherein the first and second multi-motordrive units have asymmetric gear ratios.

In another aspect, an electric axle system is provided that comprises afirst dual-motor drive unit including a first planetary gear reductionthat receives power from a first pair of motors through a ring and a sungear and delivers power to a first set of axle shafts via a carrier; anda second dual-motor drive unit including a second planetary gearreduction that receives power from a second pair of motors through aring and a sun gear and delivers power to a second set of axle shaftsvia a carrier; wherein the first dual-motor drive unit has a greatergear ratio than the second dual-motor drive unit.

In yet another aspect, a method for operation of an electric drivesystem is provided that comprises while the electric drive system isoperated under a first condition, coordinating operation of a firstelectric drive unit and a second electric drive unit based on a firstpower request; wherein the first electric drive unit includes a firstplanetary gear reduction that delivers power from a first pair of motorsto a first set of axle shafts; wherein the second electric drive unitincludes a second planetary gear reduction that delivers power from asecond pair of motors to a second set of axle shafts; and wherein thefirst and second electric drive units have asymmetric gear ratios. Inone example, the method may further comprise when vehicle speed isgreater than a first threshold value, operating one of the first andsecond electric drive units based on a second power request. In anotherexample the method may further comprise, while the electric drive systemis operated under a second condition, operating the first electric driveunit while the second electric drive unit is inoperative based on asecond power request less than the first power request. In yet anotherexample, the method may further comprise disconnecting the firstelectric drive unit through operation of a disconnect clutch, whereinthe first electric drive unit has a higher gear ratio than the secondelectric drive unit. Still further in another example, the method mayfurther comprise partially disconnecting the first electric drive unitthrough operation of a disconnect clutch based on load splitting ratiobetween the first electric drive unit and the second electric driveunit.

In yet another aspect, a continuously variable electric drive system isprovided that comprises a first multi-motor drive unit with a firstplanetary gear reduction that delivers power from a first pair of motorsto a first drive axle; and a second multi-motor drive unit with a secondplanetary gear reduction that delivers power from a second pair ofmotors to a second drive axle; and a controller comprising: instructionsthat when executed by a processor cause the controller to: selectivelycoordinate operation of the first and second multi-motor drive unitsbased on a power request; wherein the first and second multi-motor driveunits have asymmetric gear ratios.

In another aspect, a continuously variable electric drive system isprovided that comprises a first dual-motor drive unit with a firstplanetary gear reduction that delivers power from a first pair of motorsto a first drive axle; and a second dual-motor drive unit with a secondplanetary gear reduction that delivers power from a second pair ofmotors to a second drive axle; and a controller comprising: instructionsthat when executed by a processor cause the controller to: selectivelycoordinate operation of the first and second dual-motor drive unitsbased on a power request; wherein the first and second dual-motor driveunits have asymmetric gear ratios.

In any of the aspects or combinations of the aspects, a first motor anda second motor in the first pair of motors of the first multi-motordrive unit may have a common rotational axis.

In any of the aspects or combinations of the aspects, the first motorand the second motor may be positioned on opposing axial sides of anoutput gear.

In any of the aspects or combinations of the aspects, a first motor anda second motor in the first multi-motor drive unit may have rotationalaxes that are arranged parallel to one another.

In any of the aspects or combinations of the aspects, the first andsecond motors may be arranged adjacent to one another on one axial sideof an output gear.

In any of the aspects or combinations of the aspects, the first andsecond motors may be positioned laterally between two opposing vehicleframe rails.

In any of the aspects or combinations of the aspects, a first motor anda second motor of the first pair of motors in the first multi-motordrive unit may have a common rotational axis and wherein a first motorand a second motor of the second pair of motors in the secondmulti-motor drive unit have rotational axes that are arranged parallelto one another.

In any of the aspects or combinations of the aspects, the first andsecond sets of axle shafts may be included in a non-steerable tandemdrive axle.

In any of the aspects or combinations of the aspects, the electric drivesystem may further comprise a clutch arranged between one of the firstand second multi-motor drive units and an output of the electric drivesystem.

In any of the aspects or combinations of the aspects, each of the motorsin the first and second pairs of motors may be motor-generators.

In any of the aspects or combinations of the aspects, the firstplanetary gear reduction has a ratio less than 10:1 and the secondplanetary gear reduction has a ratio greater than 10:1.

In any of the aspects or combinations of the aspects, each of the firstand second multi-motor drive units may have a continuously variable gearratio.

In any of the aspects or combinations of the aspects, a first motor anda second motor in the first dual-motor drive unit and the firstplanetary gear reduction may be coaxially arranged and an output gearmay be radially offset from the first motor, the second motor, and thefirst planetary gear reduction.

In any of the aspects or combinations of the aspects, rotational axes ofthe first pair of motors in the first dual-motor drive unit may beparallel to one another and wherein rotational axes of the second pairof motors in the second dual-motor drive unit may be parallel to oneanother.

In any of the aspects or combinations of the aspects, the first andsecond dual-motor drive units may be positioned laterally between a pairof frame rails.

In any of the aspects or combinations of the aspects, the first andsecond sets of axle shafts may be included in beam axles configured toattach to a dependent suspension system.

In any of the aspects or combinations of the aspects, the firstdual-motor drive unit may have a disconnect clutch positioned between anoutput of the first planetary gear reduction and an output gear of theelectric axle system.

In any of the aspects or combinations of the aspects, the first andsecond planetary gear reductions may be simple planetary gear reductionsand the first and second dual-motor drive units each may have acontinuously adjustable gear ratio.

In any of the aspects or combinations of the aspects, the first andsecond planetary gear reductions may not include clutches or brakes.

In any of the aspects or combinations of the aspects, coordinatingoperation of the first electric drive unit and the second electric driveunit may include coordinating operation of the first and second electricdrive units based on an efficiency of the first electric drive unit andan efficiency of the second electric drive unit.

In any of the aspects or combinations of the aspects, the first andsecond conditions may be conditions where vehicle speed is less than thefirst threshold value.

In any of the aspects or combinations of the aspects, the firstcondition may be a first launch condition where the electric drivesystem initiates operation from a zero speed value and the first powerrequest may be greater than a second threshold value.

In any of the aspects or combinations of the aspects, the secondcondition may be a second launch condition where the electric drivesystem initiates operation from the zero speed value and where the firstpower request may be less than the second threshold value.

In any of the aspects or combinations of the aspects, the controller mayfurther comprise instructions that when executed, during a second launchcondition, cause the controller to: operate the first multi-motor driveunit while the second multi-motor drive unit is inoperative; wherein thesecond launch condition is a condition where the first power request isless than a threshold value and the first launch condition is acondition where the first power request is greater than the thresholdvalue.

In any of the aspects or combinations of the aspects, the controller mayfurther comprise instructions that when executed, during a cruisecondition, cause the controller to: operate the second multi-motor driveunit while the first multi-motor drive unit is disconnected.

In any of the aspects or combinations of the aspects, the cruisecondition may be a condition when vehicle speed is above a thresholdvalue and the first power request is below a threshold value.

In any of the aspects or combinations of the aspects, operating thefirst and second multi-motor drive units in tandem based on the firstpower request, may include operating the first and second multi-motordrive units to maintain each of the first and second multi-motor driveunits within a target efficiency range.

In any of the aspects or combinations of the aspects, operating thefirst and second multi-motor drive units in tandem based on the firstpower request may include operating a disconnect clutch coupled to oneof the first and second multi-motor drive units to partially disengagetwo sets of plates in the clutch based on an efficiency set-point.

In any of the aspects or combinations of the aspects, selectivelycoordinating operation of the first and second dual-motor drive unitsbased on the power request may include: while vehicle speed is less thana first threshold value and a power request is less than a secondthreshold value, operating one of the first and second dual-motor driveunits.

In any of the aspects or combinations of the aspects, selectivelycoordinating operation of the first and second dual-motor drive unitsbased on the power request may include while vehicle speed is less thanthe first threshold value and the power request is greater than thesecond threshold value, operating the first and second dual-motor driveunits in tandem.

In any of the aspects or combinations of the aspects, whereinselectively coordinating operation of the first and second dual-motordrive units based on the power request may include while vehicle speedis greater than the first threshold value, operate one of the first andsecond dual-motor drive units while a disconnect clutch coupled to theother one of the first and second dual-motor drive units and thecorresponding drive axle is disengaged.

In any of the aspects or combinations of the aspects, selectivelycoordinating operation of the first and second dual-motor drive unitsbased on the power request may include coordinating operation of thefirst and second dual-motor drive units based on an efficiency of thefirst dual-motor drive unit and an efficiency of the second dual-motordrive unit.

In another representation, a tandem electric axle system is providedthat includes a first beam axle assembly with a first continuouslyvariable dual-motor drive unit and a second beam axle with a secondcontinuously variable dual-motor drive unit, wherein the secondcontinuously variable dual-motor drive unit has a planetary gearreduction with a greater gear ratio than a planetary gear reduction inthe first continuously variable dual-motor drive unit.

As used herein, the terms “approximately” and “substantially” areconstrued to mean plus or minus five percent of the range, unlessotherwise specified.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example, and notlimitation. It will be apparent to persons skilled in the relevant artsthat the disclosed subject matter may be embodied in other specificforms without departing from the spirit of the subject matter. Theembodiments described above are therefore to be considered in allrespects as illustrative, not restrictive.

Note that the example control and estimation routines included hereincan be used with various powertrain and/or vehicle systemconfigurations. The control methods and routines disclosed herein may bestored as executable instructions in non-transitory memory and may becarried out by the control system including the controller incombination with the various sensors, actuators, and other transmissionand/or vehicle hardware. Further, portions of the methods may bephysical actions taken in the real world to change a state of a device.The specific routines described herein may represent one or more of anynumber of processing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various actions,operations, and/or functions illustrated may be performed in thesequence illustrated, in parallel, or in some cases omitted. Likewise,the order of processing is not necessarily required to achieve thefeatures and advantages of the example examples described herein, but isprovided for ease of illustration and description. One or more of theillustrated actions, operations and/or functions may be repeatedlyperformed depending on the particular strategy being used. Further, thedescribed actions, operations and/or functions may graphically representcode to be programmed into non-transitory memory of the computerreadable storage medium in the vehicle and/or electric drive system,where the described actions are carried out by executing theinstructions in a system including the various hardware components incombination with the electronic controller. One or more of the methodsteps described herein may be omitted if desired.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific examples are notto be considered in a limiting sense, because numerous variations arepossible. For example, the above technology can be applied topowertrains that include different types of propulsion sources includingdifferent types of electric machines, internal combustion engines,and/or transmissions. The subject matter of the present disclosureincludes all novel and non-obvious combinations and sub-combinations ofthe various systems and configurations, and other features, functions,and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A method for operation of an electric drive system, comprising: whilethe electric drive system is operated under a first condition,coordinating operation of a first electric drive unit and a secondelectric drive unit based on a first power request; wherein the firstelectric drive unit includes a first planetary gear reduction thatdelivers power from a first pair of motors to a first set of axleshafts; wherein the second electric drive unit includes a secondplanetary gear reduction that delivers power from a second pair ofmotors to a second set of axle shafts; and wherein the first and secondelectric drive units have asymmetric gear ratios.
 2. The method of claim1, further comprising, when vehicle speed is greater than a firstthreshold value, operating one of the first and second electric driveunits based on a second power request.
 3. The method of claim 2, furthercomprising, while the electric drive system is operated under a secondcondition, operating the first electric drive unit while the secondelectric drive unit is inoperative based on a second power request lessthan the first power request.
 4. The method of claim 3, wherein thefirst and second conditions are conditions where vehicle speed is lessthan the first threshold value.
 5. The method of claim 3, wherein: thefirst condition is a first launch condition where the electric drivesystem initiates operation from a zero speed value; and the first powerrequest is greater than a second threshold value.
 6. The method of claim5, wherein: the second condition is a second launch condition where theelectric drive system initiates operation from the zero speed value; andthe first power request is less than the second threshold value.
 7. Themethod of claim 1, further comprising disconnecting the first electricdrive unit through operation of a disconnect clutch, wherein the firstelectric drive unit has a higher gear ratio than the second electricdrive unit.
 8. The method of claim 1, wherein coordinating operation ofthe first electric drive unit and the second electric drive unitincluding coordinating operation of the first and second electric driveunits based on an efficiency of the first electric drive unit and anefficiency of the second electric drive unit.
 9. The method of claim 1,further comprising partially disconnecting the first electric drive unitthrough operation of a disconnect clutch based on load splitting ratiobetween the first electric drive unit and the second electric driveunit.
 10. An electric drive system, comprising: a first multi-motordrive unit including a first planetary gear reduction that deliverspower from a first pair of motors to a first set of axle shafts; asecond multi-motor drive unit including a second planetary gearreduction that delivers power from a second pair of motors to a secondset of axle shafts; wherein the first and second multi-motor drive unitshave asymmetric gear ratios; and a controller including: instructionsthat when executed, during a first launch condition, cause thecontroller to: operate the first and second multi-motor drive units intandem based on a first power request.
 11. The electric drive system ofclaim 10, wherein the controller further comprises: instructions thatwhen executed, during a second launch condition, cause the controllerto: operate the first multi-motor drive unit while the secondmulti-motor drive unit is inoperative; wherein the second launchcondition is a condition where the first power request is less than athreshold value and the first launch condition is a condition where thefirst power request is greater than the threshold value.
 12. Theelectric drive system of claim 10, wherein the controller furthercomprises: instructions that when executed, during a cruise condition,cause the controller to: operate the second multi-motor drive unit whilethe first multi-motor drive unit is disconnected.
 13. The electric drivesystem of claim 12, wherein the cruise condition is a condition whenvehicle speed is above a threshold value and the first power request isbelow a threshold value.
 14. The electric drive system of claim 10,wherein operating the first and second multi-motor drive units intandem, based on the first power request, includes operating the firstand second multi-motor drive units to maintain each of the first andsecond multi-motor drive units within a target efficiency range.
 15. Theelectric drive system of claim 10, wherein operating the first andsecond multi-motor drive units in tandem, based on the first powerrequest, includes operating a disconnect clutch coupled to one of thefirst and second multi-motor drive units to partially disengage two setsof plates in the clutch based on an efficiency set-point.
 16. Acontinuously variable electric drive system, comprising: a firstdual-motor drive unit with a first planetary gear reduction thatdelivers power from a first pair of motors to a first drive axle; asecond dual-motor drive unit with a second planetary gear reduction thatdelivers power from a second pair of motors to a second drive axle; anda controller comprising: instructions that when executed by a processorcause the controller to: selectively coordinate operation of the firstand second dual-motor drive units based on a power request; wherein thefirst and second dual-motor drive units have asymmetric gear ratios. 17.The continuously variable electric drive system of claim 16, whereinselectively coordinating operation of the first and second dual-motordrive units based on the power request includes: while vehicle speed isless than a first threshold value and a power request is less than asecond threshold value, operating one of the first and second dual-motordrive units.
 18. The continuously variable electric drive system ofclaim 17, wherein selectively coordinating operation of the first andsecond dual-motor drive units based on the power request includes: whilevehicle speed is less than the first threshold value and the powerrequest is greater than the second threshold value, operating the firstand second dual-motor drive units in tandem.
 19. The continuouslyvariable electric drive system of claim 17, wherein selectivelycoordinating operation of the first and second dual-motor drive unitsbased on the power request includes: while vehicle speed is greater thanthe first threshold value, operate one of the first and seconddual-motor drive units while a disconnect clutch coupled to the otherone of the first and second dual-motor drive units and the correspondingdrive axle is disengaged.
 20. The continuously variable electric drivesystem of claim 16, wherein selectively coordinating operation of thefirst and second dual-motor drive units based on the power requestincludes: coordinating operation of the first and second dual-motordrive units based on an efficiency of the first dual-motor drive unitand an efficiency of the second dual-motor drive unit.